Motion sensor

ABSTRACT

A motion sensor, and an implantable cardiac stimulator incorporating such a motion sensor, has a fluid-type housing containing a fluid that includes at least one type of anisotropic molecules, the anisotropic molecules exhibiting an anisotropic property having a state that changes dependent on motion. The housing of the motion sensor is located externally on an animate subject, or is implanted in the animate subject, and includes externally accessible electrodes that detect a change in the state of the anisotropic property and emit an output signal representative of an activity level of the subject.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a motion sensor for measuring a patient'sactivity level of the type having a fluid-tight, biocompatible housing,a number of electrodes coupled to the housing and the housing containinga fluid. Furthermore, the invention relates to an implantable cardiacpacemaker incorporating such a motion sensor.

2. Description of the Prior Art

Pacemakers are used to pace the beating frequency of the heart. Theyhave been of greatest importance for helping individuals suffering fromvarious heart diseases and/or failures to live rather normal lives.Pacemakers function by generating electrical pulses, which stimulate theheart. In order to pace the heart in a correct way, the specific pulsepattern of the individual heart must be known. It is advantageous todetect external parameters, such as the motion of the patient, in orderfor the pacemaker to stimulate the heart as correctly as possible.

Normally, the motion of the patient is measured by some kind of motionsensor, which is connected to the pacemaker. U.S. Pat. No. 5,755,741discloses an implantable sensor providing an indication of movement andorientation of a patient. The sensor may be connected to a pacemaker.The sensor has a cylindrical enclosure having a central electrode withina cavity of the enclosure, and one or more peripheral electrodes withinthe cavity. An electrolytic fluid is positioned in the enclosure so thatmovement of the sensor results in variations of the amount of the fluidbetween the central electrode and one or more of the peripheralelectrodes. An alternating current applied to the electrodes willproduce an output voltage signal, which varies in accordance with themovements of the fluid (and the sensor).

U.S. Pat. No. 5,833,713 discloses an accelerometer-based, multi-axisphysical activity sensor for use with a rate-responsive pacemaker. Apiezoelectric polymer film is adhered to the surface of an electricallyconductive substrate on the sensor. In response to bodily accelerationsthe piezoelectric film produces an output signal.

U.S. Pat. No. 5,233,984 discloses a multi-axis sensor, for exampleconnected to a pacemaker, for measuring a patient's activity level. Thesensor has a hermetically sealed, fluid-tight, biocompatible housingwith a number of electrodes coupled to the sides of the housing, and acentral electrode positioned within the housing. An electricallyconductive electrolyte fills about half of the housing, allowing voltagechanges, due to motion of the sensor, between the central electrode andthe other electrodes to be monitored.

Also, U.S. Pat. No. 4,869,251 discloses a pacemaker having a sensor fordetecting inertial and/or rotational movements of a patient. The sensorhas a hollow member, with at least one freely moveable member thereingenerating a mechanical vibration upon movement of the patient. Atransducer generates an electric signal corresponding to the mechanicalvibrations. It is stated that the hollow member may be filled with afluid and/or a number of particles producing pressure to the walls ofthe hollow member. It is also stated that the sensitivity of the sensormay be different in different directions. For example, the moveableelement may be a magnetic dipole, and the transducer may be one or morecoils arranged around the hollow member, whereby a current is generatedin the coils when the moveable element changes position in the interiorof the hollow member. Depending on configuration, however, the mobilityof the movable element may degrade due to mechanical wear.

Such known motion sensors generally rely on moving mechanicalcomponents, and therefore they may be difficult to miniaturize further.This is a problem, because the size of the motion sensor (andpacemakers) advantageously is as small as possible, in order to disturbthe biological system into which they are implanted as little aspossible. Furthermore, as mentioned above, mechanically based sensorsmay suffer from mechanical wear, which can lead to their failure.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amotion sensor that overcomes the drawbacks of the prior art, and has asfew mechanical parts as possible, as well as the sensor being as smallas possible, and relying on a technical principle making it as resistantto failure as possible.

These and other objects are accomplished by a motion sensor according tothe present invention, which relies on the principle of an electricallydetectable anisotropic fluid, which fluid orients itself in relation toexternal motion.

A sensor thus is achieved employing no moving parts, and which thereforecan be made very small.

In a preferred embodiment, the anisotropic fluid contains long, rigidLCP-molecules, on which electrically detectable magnetic nanoparticlesare covalently linked.

In another embodiment a magnetic field is applied to the anisotropicfluid, and the alignment of the anisotropic molecules is detected bymeasuring the capacitance of the fluid.

In yet another embodiment an electrostatic field is applied to theanisotropic fluid, and the alignment of the anisotropic molecules isdetected by measuring the capacitance of the fluid.

Moreover, the invention relates to an electrically detectableanisotropic fluid comprising a liquid crystalline polymer (LCP) as theanisotropic fluid, which LCP is covalently bound to an iron-oxidenanoparticle. This fluid is specifically suitable for use in the motionsensor of the invention.

Furthermore, the invention relates to an implantable pacemakerincorporating the motion sensor of the invention. A patient's heart ispaced as a response to the patient's motion, sensed by the motion sensorof the invention.

Accordingly, the drawbacks of the prior art are overcome.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a motion sensor according to one embodimentof the invention.

FIG. 2 is a block diagram of a rate response pacemaker accordingly tothe present invention, incorporating a motion sensor according to thepresent invention.

FIG. 3 shows a zwitterion.

FIG. 4 is a block diagram of a motion sensor according to anotherembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aim of the invention is to provide a motion sensor, especiallyadapted for use with a pacemaker, which is as small as possible, andwhich circumvents the problem of mechanical wear in a sensor comprisingseveral mechanical parts. This is accomplished by a sensor that relieson the principle of an anisotropic fluid or an anisotropic moleculewithin a fluid. Thus, the only moving parts of the sensor are fluids.

In accordance with the invention a motion sensor for measuring apatient's activity level has a fluid-tight, bio-compatible housing, anumber of electrodes coupled to the housing, and the housing contains afluid that includes at least one anisotropic molecule, the anisotropicproperties of which change in relation to the motion of the fluid, sothe state of the anisotropic molecules of the fluid is detectable by theelectrodes.

In a preferred embodiment, the motion sensor is implantable in apatient. The motion sensor alternatively may be adapted for externaluse, i.e. not being implanted in the body of the patient, butcommunicating with an implanted pacemaker, such as by telemetry.

The term “patient” as used herein means an animal, especially a human,in need of a motion sensor. This may include, for example, personshaving a pacemaker or who are to be equipped with a pacemaker. Themotion sensor of the invention, however, need not necessarily be coupledto a pacemaker, but may be connected to any other device or circuitryfor which the output of the motion sensor is relevant.

As used herein, a patient's “activity level” means the degree of motionfor the specific patient. For example, for a pacemaker to deliver thecorrect pulses to a patient's heart, it is important to correlate thesepulses with the “activity level” of the patient, in order for the pacingpulses and the resulting heart responses to be suited for the patientspresent need. Thus, it is preferred that the paced heart rate isadaptable to different levels of exertion by the patient.

As used herein, “a number of electrodes” means at least one electrodepair, having the capacity to monitor at least one electronic parameter.

An anisotropic molecule will orient itself in relation to motion-causedforces to which it is subjected. Thus, for example, an anisotropicmolecule may exhibit anisotropic properties at rest and isotropicproperties when agitated (i.e. different states). Accordingly, a numberof molecules of this kind will, like a crystal, align in a commondirection at rest, or in a laminar shearing force. The viscosity, aswell as several other physico-chemical properties, such as opticaltransmission, heat transfer, polarity, conductivity etc., will then bevery different in the alignment direction compared to other directions,especially the perpendicular direction. However, if agitated, themolecular alignments will become random, and as a result properties suchas viscosity or dielectric constant will be practically equal in alldirections. Thus, when a patient carrying the sensor of the inventionmoves, the sensor will sense these movements (or lack of movements) andcreate an output signal dependent on the movements.

Liquid crystalline polymers (LCP's)(Langmuir 2001, 17, 2900-2906) arefluids having anisotropic properties at rest and isotropic propertiesduring agitation. LCP's, in the context of the invention, are long rigidmolecules, which have an aspect-ratio (length/diameter) of greater thanabout (10/1). One example, which is suitable for the purposes of theinvention, is poly-(p-phenylene) with a degree of polymerization (n)equal to or greater than 10. In this context, the aspect-ratio would bea more relevant term than for example molecule length, since a rigid rodpolymer with a wide diameter will not exhibit anisotropic properties atthe same length or degree of polymerization as a rigid rod polymer witha narrow diameter. The aspect-ratio, however, provides a good way tomeasure the intrinsic anisotropy of the polymer molecules.

Thus, according to one embodiment of the invention the anisotropicmolecule is a liquid crystalline polymer (LCP).

Some other examples of liquid crystalline polymers suitable for theinvention are given below. However, this list is not exhaustive, andother compounds may also be used.

A copolymer of p-hydroxybenzoic acid and polyethylene terephthalate.(Japanese Patent Publication No. 18016/1981); a copolymer ofp-hydroxybenzoic acid, polyethylene terephthalate, aromatic diol (suchas 4,4′-dihydroxybiphenyl), and aromatic dicarboxylic acid, withimproved flowability and heat resistance. (Japanese Patent Laid-open No.30523/1988); a copolymer of p-hydroxybenzoic acid,4,4′-dihydroxybiphenyl, t-butylhydroquinone, and terephthalic acid.(Japanese Patent Laid-open No. 164719/1987); a copolymer ofp-hydroxybenzoic acid, 4,4′-dihydroxybiphenyl, isophthalic acid, andterephthalic acid. (Japanese Patent Publication No. 24407/1982 andJapanese Patent Laid-open No. 25046/1985); a copolymer ofp-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. (Japanese PatentLaid-open No. 77691/1979); copolyesters of terephthalic acid,alkylhydroquinone, p-hydroxybenzoic acid and poly(alkyleneterephthalate), the alkylene substituent preferably containing ethyleneor butylene and the alkyl substituent of the hydroquinone preferablycontaining a lower alkyl group such as propyl or (tertiary) butyl;copolyesters of p-hydroxybenzoic acid and poly(alkylene terephthalate),the alkylene group preferably being ethylene or butylene; copolyestersof terephthalic acid, alkylhydroquinone, p-hydroxybenzoic acid andhydroxyalkylphenyl-alkanoic acids, the alkyl-substituent of thehydroquinone preferably containing a lower alkyl group such as propyl or(tertiary) butyl, the alkanoic acid preferably containing 3 to 8 carbonatoms, propanoic acid being particularly preferred, andblockcopolyesters of trimellithic imide-terminated poly(THF) orpolysilicone, containing the imide group in para- or meta-position i.e.N-(4-carboxy-phenyl)-trimellit imide or N-(3′-acetoxy-phenyl)-trimellitimide, with acetoxybenzoic acid and at least one repeating unit selectedfrom the group including diacetoxy diphenyl, hydroquinone diacetate,terephthalic acid, a trimer designated HBA-HQ-HBA (the synthesis ofwhich is described in Europ. Polym. J. 20, 3, 225-235 (1984), andpoly(ethylene terephthalate) (PET).

The molecular weight of the liquid crystal polymer used in the presentinvention depends on the character of the repeating units of the LCP.Usually, the molecular weight is in the range of about 1,000 to 300,000.If fully aromatic polyesters are used as LCP's, their molecular weightis typically in the range of about 2,000 to 200,000, preferably about10,000 to 50,000.

More general details on liquid crystalline polymers and their propertiesand applications are given in an article titled “Liquid Crystal Polymersand Their Applications” by Chung et al. in Handbook of Polymer Scienceand Technology, Vol. 2 (1989) 625-675.

In order to be able to detect the anisotropic properties a doping agent,for example, may be attached to the anisotropic molecule, the dopingagent having the capacity to provide a readable electronic signal. Thus,the doping agent constitutes an electrically detectable componentallowing a patient's activity level to be monitored.

The doping agent is coupled to the LCP-molecule by any common chemicalbonding technique. Preferably, the doping agent is coupled to theLCP-molecule by covalent bonding. Those skilled in the art are familiarwith techniques for binding the doping agent to the LCP, e.g. byhydrosilylation reactions using organosilane intermediates, for exampledimethylvinyl silane.

Therefore, according to another embodiment the anisotropic moleculeincludes an electrically detectable component.

The production of silica-coated iron oxide nanoparticles is described inthe reference article Langmuir, Vol 17, No. 10, 2001 (2900-2906).Basically, they are produced in solution from appropriate chemicalreagents, aided by suspension in organic surfactants. The particles aretreated with tetraethylorthosilicate (TEOS) to form a silica coating.These particles are magnetic, and can be bound to an isotropic (rigidrod) molecule using covalent bonding and standard silica surfacetreatment chemistry, for example by silanization with dimetylvinylsilane. In one embodiment of the present invention, the silane groupbonds to the silica surface, and the vinyl group can form a covalentbond with many different kinds of organic molecules, including LiquidCrystalline Polymers (LCP's) containing dipoles. Under a small magneticfield (B), such as greater than about 0.5 Gauss, a preferentialorientation will be induced, so that the magnetic particles line up inthe magnetic field. The degree of alignment is preferably detected bymeasurement of capacitance across the fluid. The tendency for alignmentis dependent on agitation because the dipoles will be forced by themagnetic particles to line up in the preferential orientation, but thatorientation is disturbed by fluid motion.

A basic illustration of one embodiment of the motion sensor of theinvention is shown in FIG. 1. The sensor 10 has a housing 11, which isfilled by a fluid. At least a part of the fluid is anisotropic moleculesof the invention. Furthermore, a magnetic field is generated by themagnets 14 and 15. The fluid forms a capacitor with electrodes 12 and 13the capacitance of which can be measured, and thus a variation in theanisotropic status of the fluid is monitored.

Examples of doping agents are ions or molecular dipoles. Preferably, amagnetically aligned nanoparticle (Langmuir 2001, 17, 2900-2906), or acharge-separated ion-pair, such as a zwifterionic pair (positive chargeat one end, negative at the other) is used. The nanoparticle is in apreferred embodiment an iron-oxide nanoparticle, wherein iron-oxide maybe represented by e.g. Fe₃O₄, or γ-Fe₂O₃ (Langmuir 2001, 17, 2900-2906)

Thus, in another embodiment of the invention the detectable component isa magnetic nanoparticle, a zwitterionic pair or a charge separatedion-pair, preferably a magnetic iron-oxide nanoparticle.

In one preferred embodiment, the anisotropic fluid contains long rigidLCP-molecules, on which electrically detectable charge-separated ionpairs are covalently linked.

In another embodiment an electric field is applied to the anisotropicfluid, and alignment of the anisotropic molecules is detected bymeasuring the capacitance of the fluid.

Moreover, the invention relates to an electrically detectableanisotropic fluid comprising a LCP as the anisotropic fluid, which LCPis covalently bound to two charge-separated ion pairs of differentcharge at each end of the LCP rigid rod.

In a non-polar medium such as an LCP fluid, ions are associated withother ions of opposite charge, forming an ion pair. One ion pair aloneis sufficient to cause orientation in an electric field. However, a verystrong field would be required to align long rod LCPs if only one ionpair were attached at one end. Consequently, it is preferable to havetwo different ion pairs at each end (one must be a negative groupcovalently bond, such as a carboxylate ion R—COO⁻ with counter-ion Na⁺,and the other a positive ionic group such as R′—NH₃ ⁺ with counter-ionC1⁻). Choices of the covalently bonded ion and its counter-ion are many,and any choice that is suitable for use in the present invention may bechosen. A “zwitterion” is the name for any molecule containing two suchionic groups of opposite charge as described above and illustrated inFIG. 3.

In an electrostatic field, as is created between capacitor plates ofopposite charge (FIG. 4), the charges on the LCP pictured above willcause the rigid rod to readily align itself in the field, creatinganisotropy behavior in the many properties of the fluid, including (butnot limited to) viscosity, resistance, dielectric constant, etc. A verylow electrical field strength (E) may be possible to facilitate thealignment, preferably E less than 1 V/m. Agitation of the fluid causestemporary randomization of orientation which is easily detected in anumber of ways as discussed previously for the magnetic particle dopedmaterials. At rest, the anisotropy is restored to the fluid due to theinfluence of the electrical field aligning the ion pairs of thesezwitterionic molecules. The viscosity, as well as other physico-chemicalproperties, such as optical transmission, heat transfer, polarity,conductivity, etc., will then be different in the alignment directioncompared to other directions, especially the perpendicular direction.

FIG. 4 illustrates this embodiment. A motion sensor 40 has abiocompatible housing 41 and electrodes 42 and 43. Positively andnegatively charged capacitor plates, 44 and 45, respectively, are usedto align the LCP-molecules in the housing.

Those skilled in the art (of combinatorial organic chemistry, forexample) are familiar with techniques for binding the doping agent tothe LCP, e,g, by hydrosilylation reactions involving organosilaneintermediates, for example dimethylvinylsilane, which leaves a vinylgroup for binding e.g. organic acid and base groups such as —R—COO⁻ and—R′—NH₃ ⁺ where R and R′ are any organic moiety containing a vinylgroup.

A reason for the use of a zwitterionic or charge-separated ion pair as adopant on the LCP-molecule, and the use of an electrostatic field (suchas between capacitor plates) for alignment of the LCP-molecules, is thatit is desirable to avoid disturbances of the function of a pacemakerfrom external magnetic fields, and this need is met by this embodiment.

The electrical/magnetic signature is representative of the degree ofalignment of the LCPs. As a consequence, the amount of agitation (i.e.patient activity) is correlated to the electrical or magnetic signal,which the sensor and/or pacemaker circuitry is designed to detect. As aresult a motion sensor having no moving parts (except the anisotropicfluid) and that can be made very small can be manufactured. For example,the sensor may have a cavity volume of a few mm ³ or less, such as lessthan 10 mm³, preferably less than 5 mm³, most preferably less than 1mm³.

The suitable electronic signal to be monitored may vary depending on thenature of the system and its chemical content. However, for manyapplications measure of capacitance is suitable. In other casesmeasurement of voltage, voltage changes, impedance or resistance may besuitable.

Thus, in another embodiment the state of the anisotropic molecule ismonitored by measuring the capacitance of the fluid.

In another embodiment the state of the anisotropic molecule is monitoredby measuring the resistance of the fluid.

In another embodiment, coils are arranged around the housing. Upon avariation in the state of the anisotropic fluid, a voltage is induced inthe coils, which voltage is used as a signal. For example, the coil maybe of insulated silver/copper wire.

The nanoparticulate size of the anisotropic entity molecules enables lowenough viscosity that no moving parts (other than fluid) is necessary,even in small devices like a pacemaker. A half-empty cylinder is onepreferred example of the container. This container preferably has amagnetic N and a magnetic S pole provided by placement of permanentmagnets 14 and 15 on opposite sides of its diameter. The electric sensor(e.g. capacitor electrodes 12 and 13) may also be on opposite sides ofthe cylinder diameter, though preferably normal (90 degrees) to themagnetic diameter (see FIG. 1).

It is not required to have the capacitor electrodes normal to themagnetic field, but it provides the optimum signal output. Each moleculeof the fluid will act as a dipole with a dipole moment in a particulardirection. When in a magnetic field, the dipole moments will tend toalign in due to being bound to the magnetic particles as described inthe reference article. The net dipole moment of the fluid is then finiteand detectable as capacitance between the capacitance electrode. This“high order state” is detectable across any plane of the fluid exceptthat parallel to the magnetic field, since in the parallel plane the LCPmolecule ends face the electrodes and the LCP fluid would appear to thesensor to be small molecules with no preferable orientation in thatplane. Therefore, in the “high order state”, the signal is obtained aslong as the capacitor electrodes are not in the parallel plane, adetectable signal in any other plane, and an optimum signal is obtainedin the perpendicular plane. Note that when agitated, the dipoles becomerandomly oriented yielding a much lower net dipole moment—this “loworder state” gives the same signal output independent of orientationbetween the capacitor electrodes and the magnetic field. So to achievethe maximum difference between the “at rest” state (high order) and theactive state (low order), perpendicular is the preferred set-up, butsensitive electronics means that any orientation other than parallel tothe magnetic field suffices.

However, in another embodiment an anisotropic molecule exhibiting aspecific dipole moment even in the parallel plane is used.

In the invention, air is preferably used as the agitating medium.However, it is also possible to have a completely filled cavity if it isensured that the anisotropic fluid is agitated with body motion (note,it is fluid agitation that results in its altered properties). One suchembodiment is a flexible housing completely filled, whereby the motionof the housing provides the agitation. Another embodiment would be tohave a completely filled cavity containing inert particles in the fluid,which agitate the fluid with body motion (kind of like the ball bearingin spray-paint cans). If air is the agitator (which is preferred), 98%fluid is a reasonable upper limit. As a lower limit, enough fluid tofill the space between electrodes (preferably capacitor plates) isneeded, which would necessitate a lower limit of about 20% fluid.

It is important that the sensor be implantable in the human body so thesensor is preferably constructed of biocompatible material. Furthermore,since the housing of the sensor contains a fluid, it is important thatthe housing be entirely fluid-tight, in order to assure the function ofthe sensor as well as no risk of harm of the patient.

The housing may be formed, for example, of glass, ceramic, Plexiglas®,thermoplastic, curable plastic, metal, rubber (for example siliconerubber) or any other suitable material. Also, the housing may be formedof any conventional structural biocompatible dielectric material, suchas ceramic, phenolic resin, epoxy resin or polysulfone. The shape of thehousing may for example be spherical, cylindrical, cubical, of ahorseshoe-shaped annulus form, or any other functional shape.

In one embodiment, the housing is configured in the shape of a cubehaving six sides, and the electrode includes six generally identicalrectangularly shaped side electrodes. Each of these side electrodes iscoupled to one side of the housing. The electrodes are electricallyaccessible outside of the sensor via conventional feed-throughs.

The electrodes can assume different shapes, such as a rectangle, acircle, a triangle, a parabola, or such other geometric shapes that willenable the mapping of the voltages, voltage changes, impedances andimpedance changes between various reference points on the sideelectrodes.

These side electrodes are composed of conventional conductive materialsuch as stainless steel or titanium. The side electrodes may besupported remotely by feed-through connector wires or may be connectedto the side via an insulator.

In another embodiment shear forces acting on in the anisotropic fluidare produced by using a solid mechanical moveable element, such as abead or a ball made of non-ferromagnetic metal, contained in thehousing.

The sensitivity of the sensor can be varied by varying the concentrationof the anisotropic molecule in the fluid.

The sensitivity of the sensor may also be varied by varying theviscosity of the fluid in the housing.

The invention also encompasses an electrically detectable anisotropicfluid containing a liquid crystalline polymer (LCP) as the anisotropicfluid, which LCP is covalently bound to an iron-oxide nanoparticle.Thus, a fluid is provided that is specifically adapted for use in amotion sensor of the invention.

The invention also encompasses an implantable cardiac pacemakerincorporating the implantable motion sensor described above, and meansfor pacing a heart, such as a pulse generator, as a response to theactivity level as detected by the motion sensor, which means isconnected to said motion sensor. The physical activity of the patient inwhich the pacemaker is implanted is monitored using the sensor, and theoutput signal of the sensor is used to control the frequency of thepulse generator.

In an alternative embodiment of this aspect, the pacemaker does notcontain the motion sensor itself, but contains a unit for receivingsignals from an external motion sensor. In this case, the externalmotion sensor communicates by means of telemetry or the like with thepacemaker.

As used herein “implantable pacemaker” means a pacemaker that can besafely implanted in a patient, and thus the pacemaker should beconstructed of biocompatible material.

The term “means for pacing a heart” means any kind of conventionalpacemaker or pulse generator, such as implantable cardiacdefibrillators, such as Microny® or Regency®. Those skilled in the artknow suitable devices for use for this purpose.

In one embodiment, the sensor forms an entirely self-contained systemwithin the heart pacemaker, which requires no additional externaldetectors or line connections outside of the heart pacemaker housing.

In another embodiment, the sensor can be implanted independently,remotely from the implanted medical device. Yet another alternative isto have the patient wear the sensor externally, such that the outputsignals from the sensor are transmitted by telemetry to the implantedmedical device.

In any case, the sensor should be as small as possible, e.g. to allowincorporation into an implantable pacemaker, preferably 0.5 cm square,or less.

In a preferred embodiment the output signal is capacitance, which mustfirst be converted to a voltage signal. The sensor response frequenciesneed to be calibrated to correspond with appropriate patient motionfrequencies.

The use of a sensor of the present invention in combination with controlcircuitry for a heart pacemaker may be as follows (FIG. 2): The sensor20 according to this embodiment of the invention contains a fluid, whoseproperties of dielectric permittivity change when agitated, causing acorresponding change in the capacitance between two electrodes (asdescribed above). The variable capacitance is used as an input signal toan oscillator 21, which produces an output frequency that depends on thecapacitance input. The frequency generated by the oscillator 21 isprocessed and converted to an appropriate signal in afrequency-to-activity-signal converter 22, as is known in the art. Thesignal from the converter 22 is combined with input of a programmabletarget rate 31 and is further processed in a reaction and recoverycircuit 23 to ensure appropriate reaction time and recovery time of thestimulation to the detected activity signals. The remaining componentsare standard and known to those skilled in the art of rate-responsivecardiac pacing. Those components include a pacemaker logic circuit 24,an output circuit 25, an ECG (electrocardiogram) filter and amplifier26, a refractory unit 27, and a timer 28 for the highest inhibited rate.The total circuit is coupled to the heart 30 of a subject between theoutput circuit 25 and the ECG filter and amplifier 26. Reference is madeto e.g. U.S. Pat. No. 5,233,984 and to Lindgren and Jansson, “HeartPhysiology and Stimulation, an introduction”, 1992, Siemens-Elema AB,Solna, Sweden.

The control signal for the heart pacemaker may be generated in digitalform instead of the analog format discussed above. Digital processingcan be undertaken in a microprocessor. In both cases, the programmingcan be done via a telemetry link between the heart pacemaker and anexternal programming means.

In one embodiment of the invention a low/high frequency band pass filter(for measuring a patient's posture and activity) is used. This filterpreferably is calibrated, since the fluid properties of the fluid of thesensor will affect sensor response frequencies. Filtering frequenciesmay therefore need adjustment.

Although modifications and changes may be suggested by those skilled inthe art, it is the invention of the inventor to embody within the patentwarranted heron all changes and modifications as reasonably and properlycome within the scope of his contribution to the art.

1. A motion sensor for measuring an activity level of an animatesubject, comprising: a substantially non-deformable fluid-tight housingconfigured for placement relative to a subject for co-movement withmovements of the subject; a fluid contained in said housing, said fluidcomprising at least one type of anisotropic molecules, having ananisotropic property that changes dependent on motion of said fluidimparted to said fluid exclusively by the co-movement of said housingwith said movements of said subject; and electrodes in communicationwith said anisotropic molecules that detect a state of said anisotropicproperty, said electrodes being accessible from an exterior of saidhousing to provide an output signal representing an activity level ofthe subject.
 2. A motion sensor as claimed in claim 1 wherein saidhousing is comprised of biocompatible material, and is adapted forimplantation in the subject.
 3. A motion sensor as claimed in claim 1wherein said anisotropic molecules comprise a liquid crystallinepolymer.
 4. A motion sensor as claimed in claim 3 wherein said liquidcrystalline polymer is poly (p-phenylene) having a degree ofpolymerization equal to or greater than
 10. 5. A motion sensor asclaimed in claim 1 wherein said anisotropic molecules comprise anelectrically detectable component.
 6. A motion sensor as claimed inclaim 5 wherein said electrically detectable component is covalentlycoupled to said anisotropic molecules.
 7. A motion sensor as claimed inclaim 5 wherein said electrically detectable component is selected fromthe group consisting of magnetic nanoparticles, zwitterionic pairs, andcharge-separated ion pairs.
 8. A motion sensor as claimed in claim 5wherein said electrically detectable component comprises iron oxidenanoparticles.
 9. A motion sensor as claimed in claim 1 comprising amagnetic field source disposed externally of said housing that generatesa magnetic field that interacts with said anisotropic molecules to causesaid anisotropic property to be in an initial state, and wherein saidelectrodes detect deviation of said anisotropic property from saidinitial state.
 10. A motion sensor as claimed in claim 9 wherein saidanisotropic property is capacitance, and wherein said electrodescomprise a pair of capacitor electrodes with said fluid disposedtherebetween, said capacitor electrodes being oriented perpendicularlyto an applied direction of said magnetic field.
 11. A motion sensor asclaimed in claim 1 comprising a electrostatic field source disposedexternally of said housing that generates a electrostatic field thatinteracts with said anisotropic molecules to cause said anisotropicproperty to be in an initial state, and wherein said electrodes detectdeviation of said anisotropic property from said initial state.
 12. Amotion sensor as claimed in claim 1 wherein said anisotropic property iscapacitance, and wherein said electrodes detect the capacitance of saidfluid.
 13. A motion sensor as claimed in claim 1 wherein saidanisotropic property is resistance, and wherein said electrodes detectthe resistance of said fluid.
 14. A motion sensor as claimed in claim 1wherein said housing contains an element interacting with said fluidthat produces shear forces in said fluid that alters said anisotropicproperty of said molecules.
 15. An electrically detectable anisotropicfluid comprising a liquid crystalline polymer having moleculescovalently bound to an iron-oxide nanoparticle.
 16. A cardiac stimulatorcomprising: a motion sensor that measures an activity level of ananimate subject comprising a substantially non-deformable fluid-tighthousing configured for placement relative to a subject for co-movementwith movements of the subject, a fluid contained in said housing, saidfluid comprising at least one type of anisotropic molecules, having ananisotropic property that changes dependent on motion of said fluidimparted to said fluid exclusively by the co-movement of said housingwith said movements of said subject, and electrodes in communicationwith said anisotropic molecules that detect a state of said anisotropicproperty, said electrodes being accessible from an exterior of saidhousing to provide an output signal representing an activity level ofthe subject; a stimulator housing configured for implantation in thesubject; stimulation circuitry contained in said stimulator housing thatgenerates electrical stimulation therapy signals; an electrode systemconfigured for implantation in the subject, said electrode system beingconnected to said stimulation generator and being configured to interactwith tissue in the subject to deliver said electrical stimulationtherapy; and a control unit in said stimulator housing connected to saidstimulation generator, and being in communication with said motionsensor to receive said output therefrom representing said activitylevel, said control unit modifying said electrical stimulation therapydependent on said activity level.
 17. A cardiac stimulator as claimed inclaim 16 wherein said housing of said motion sensor is contained in saidstimulator housing.
 18. A cardiac stimulator as claimed in claim 16wherein said stimulation generator comprises a pacing pulse generator.